This disclosure relates to sewing machines in general and more particularly to sewing machines which utilize a light source and a light sensor as part of a device to advise an operator of the impending depletion of bobbin thread.
It is known in the prior art to have systems for detecting bobbin thread depletion which make use of a light source and a light sensor. Such a system is disclosed in U.S. patent application Ser. No. 916,614, filed on June 19, 1978, which discloses an adjustable bobbin thread runout indicator.
It is also known in the field of sewing machines that the bobbin-looptaker area of the sewing machines is subject to lint build up since it is in this area that the work material and threads are concentrated. Considerable ingenuity has been exercised in solving the problem of lint build up in this particular area of sewing machines. An example of one such accommodation for this purpose is found in the U.S. Pat. No. 3,083,659, issued on Apr. 2, 1963 to Kuhar. That patent teaches the use of air currents to be generated by a specially constructed rotating hook in a deliberate effort to prevent the accummulation of lint in that area. The U.S. Pat. Nos. 3,650,230 and 3,374,757 of Nishikawa and Hamlett are further examples of appliances developed to remove dust or lint from the shuttle race area of the sewing machines.
While the lint in the prior art patent created a problem in loop formation and the dust in the shuttle race created friction and binding problem, it is known to those skilled in the art that the lint and the dust may also cause a problem with the bobbin thread alarm utilizing light detectors in that the lint or the dust may mask an empty condition of the bobbin by interfering with light transmission. What is required is a self cleaning arrangement for a low bobbin thread detection system using a light sensor, in order to decrease or eliminate a sensitivity to lint accumulation.